1. Field of the Invention
The present invention relates to a solid/liquid determination apparatus by which a state of an object, whether it is solid or liquid, is determined for the purpose of heating, and furthermore relates to an automatic oven comprising the solid/liquid determination apparatus.
2. Description of the Related Art
In a conventional automatic oven such as an electric oven or a microwave oven, for example, a weight of a food mounted in the oven is detected, and a heating time of the food is decided according to the detected weight. In another example of a conventional oven, steam emanated from the food is detected during heating operation, and when a predetermined amount of emanation of the steam is detected, the heating operation is completed. In such oven, the food is heated regardless of the state of the food whether the food is solid or liquid. As a result, a temperature or a finished state of the food is influenced by the state, solid or liquid, of the food.
In the apparatus in which the weight of the food is detected and the heating time is determined in accordance with the weight, even after microwave heating of the food of the same weight for the same length of time, the temperature of the finished state of boiled rice of a solid food is different from that of soup of a liquid food. FIG. 19 is a diagram representing a relation between heating time and temperature of the boiled rice or the soup. In this case, a heating object of 130 grams of the boiled rice or 130 grams of the soup is put in each cup of 70 grams of weight, and a cup of 200 grams filled with the boiled rice or the soup is heated in a microwave oven. Referring to FIG. 19, the heating time which is required to heat the object to about 77.degree. C. (centigrade scale) is about 75 seconds in the case of the boiled rice as shown on a curve R, and the time is about 120 seconds in the case of the soup as shown on a curve MS. The soup requires as about 1.6 times long as the heating time of the boiled rice. The reason why the temperature rise of the soup is lower than that of the boiled rice is generally considered that the microwave concentrates on the surface of the soup or is reflected thereby due to the influences of a dielectric constant, an electric resistance, and/or an ionic conductivity by salt in the soup. It is considered that the such influences are small in the case of the boiled rice, and accordingly, the microwave penetrates into the boiled rice and a relatively large temperature rise is realized in comparison with the soup.
In general, a preferable temperature of the soup is about 77.degree. C. and the preferable temperature of the boiled rice is about 90.degree. C. Referring to FIG. 19, the heating times required to heat to the respective preferable temperatures are about 120 seconds for the soup as shown on the curve MS and about 90 seconds for the boiled rice as shown on the curve R. As mentioned above, speeds of temperature rises are different depending on the state of the food in the same weight, and thus the heating time for the boiled rice is different from the heating time of the soup to reach each optimum temperature.
Subsequently, the conventional automatic oven which is controlled by sensing steam emanated from a food by a steam sensor is described by taking the examples of the boiled rice and the soup. FIG. 20 is a diagram representing relation between the heating time and detected value of the steam sensor which detects the steam emanated from the boiled rice or the soup in heating operation. Small circles on curves R and MS represent heating times at the respective optimum temperatures. Referring to FIG. 20, the boiled rice emanates only small amount of steam as shown by the curve R. On the contrary, in the case of soup, an emanation of steam states immediately after a heating operation is started, and the emanation of steam greatly increases after about 85 seconds of heating time as shown by the curve MS. It is considered that the microwave concentrates on the surface of the soup, and the soup is heated in the vicinity of the surface. Consequently, the emanation of steam greatly increases. In the case of the boiled rice, the microwave penetrates into the boiled rice and diffuses therein, and consequently, the boiled rice is uniformly heated. Therefore, the emanation of the steam of the boiled rice lags behind that of the soup. For example, an output level of the steam sensor is about 0.12 volts when the boiled rice has been heated to the optimum temperature, and is about 0.58 volts when the soup has been heated to the optimum temperature. Therefore, control of the food temperature can not be realized by detecting the steam emanated from the food. Namely, in order to heat various foods to the respective preferable temperatures by the automatic oven, the heating time must be set according to the state of the food, whether it is solid or liquid. However, the above-mentioned conventional automatic ovens do not comprise really useful means for automatically determining the state of the food.
A method for detecting a solid-phase rate (it is considered to be a rate of solid state portion to liquid state portion in a melted alloy) of an alloy material in partly melted state is disclosed in the Japanese published unexamined patent application Hei 4-186145. According to the prior art, mechanical vibration is continuously applied to the alloy material in the melted state by an ultrasonic wave generator, and a variation of the vibration frequency is detected, keeping the application of the vibration to the alloy material. Then, the solid-phase rate is derived by experimental correspondence between the variation of the vibration frequency and the temperature change. The above-mentioned prior art requires a particular ultrasonic wave generator to give the mechanical vibration to the object to be measured, and such ultrasonic wave generator is expensive in cost. Furthermore, an ultrasonic sensor is generally delicate and is easily disturbed by various noises when applied to a microwave oven.
The solid/liquid determination apparatus of the present invention proposes a novel means for solving the problem in the prior art.
The inventor came to a novel concept that, without providing any particular sound or vibration generator, the difference of vibration corresponding to the states (solid or liquid) can be detected by measuring a vibration caused by stopping.